Method for processing hot-rolled metal bodies and the like

ABSTRACT

A hot-rolled metal body, e.g. a bar, rod, billet, bloom, rail or wire, emerges from between the last rolls of the rolling-mill train in a hot condition and is subjected to a high-pressure water jet to remove all traces of scale and prepare the surface of the body for receiving an enamel coating without substantial cooling of the body. The workpiece is then coated with a powder fusing at a temperature at or below the temperature of the body to form a liquid layer or film which solidifies upon cooling to provide the enamel coating. Since the enamel coating ruptures, cracks, &#39;&#39;&#39;&#39;stars&#39;&#39;&#39;&#39; or otherwise breaks in the region of surface flaws of the body, the coating serves to indicate the presence of such flaws while protecting the body against oxidation or scaling as the body cools.

United States Patent [191 Schaumburg METHOD FOR PROCESSING HOT- ROLLEDMETAL BODIES AND THE LIKE [76] Inventor: Georges Schaumburg, 1 bis ruede Londres, Montigny-Les-Metz,

France [22] Filed: Mar. 2, 1971 [21] Appl.No.: 120,084

Related US. Application Data [63] Continuation-impart of Ser. No.835,359, June 23,

1969, Pat. No. 3,648,349.

[30] Foreign Application Priority Data Nov. 14, 1970 Germany ..P 20 56135.7

[56] References Cited UNITED STATES PATENTS 8/1914 Chartener ..29/811451 Apr. 17, 1973 Primary Examiner-Charles W. Lanham AssistantExaminer-V. A. Dipalma Attorney-Karl F. Ross 57 ABSTRACT A hot-rolledmetal body, e.g. a bar, rod, billet, bloom, rail or wire, emerges frombetween the last rolls of the rolling-mill train in a hot condition andis subjected to a high-pressure water jet to remove all traces of scaleand prepare the surface of the body for receiving an enamel coatingwithout substantial cooling of the body. The workpiece is then coatedwith a powder fusing at a temperature at or below the temperature of thebody to form a liquid layer or film which solidifies upon cooling toprovide the enamel coating. Since the enamel coating ruptures,cracksfstars or otherwise breaks in the region of surface flaws of thebody, the coating serves to indicate the presence of such flaws whileprotecting the body against oxidation or scaling as the body cools.

1 1 Claims, 7 Drawing Figures PAIENTEUAPR H915 v 3, 727, 290

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A TT'UR NI'IY METHOD FOR PROCESSING HOT-ROLLED METAL BODIES AND THE LIKECROSS-REFERENCE TO COPENDING APPLICATION The instantapplication is acontinuation-in-part of application Ser. No. 835,359, filed 23 June 1969and entitled METHOD OF AND APPARATUS FOR TREATING A HOT-ROLLED METALBODY AND THE LIKE, now US. Pat. No. 3,648,349.

FIELD OF THE INVENTION My present invention relates to the production ofrolled metal bodies and, more particularly, to the treatment ofhot-rolled metal bodies. In addition, the invention relates to a methodof and a system for detecting surface flaws in hot, preferablyhot-rolled, metal bodies.

BACKGROUND OF THE INVENTION The metallurgical art provides numerousmetalforming processes among the most significant of which is, ofcourse, the hot-rolling technique. In the hotrolling of a metal body, aheated ingot, billet, bloom or slab, is passed between a pair of rollsto elongate the workpiece and flatten or shape the latter. In general,the workpiece is heated in a soaking pit or in the furnace to atemperature just below its melting point or is rolled in the hot stateat maintenance for some time after casting. A significant problem in theprocessing of heated workpieces is the formation of scale or crust uponthe body. The scale may derive from the presence of internalcontaminants but generally is a result of the interaction of the surfacemetal with ambient oxygen. Such crust or scale is relatively hard, isonly partly sloughed during rolling and in many cases must be removed bymechanical, chemical or other means to avoid damage to the rolling-millequipment or to prevent the formation of products which are unsightly ormechanically unsatisfactory.

As the body leaves the last pair of rolls in the rolling train, i.e. thesuccession of roll stands through which the workpiece is passed forstepwise reduction in its cross-section and corresponding increases ofits length, it is hot but relatively scale-free as a result of theaforementioned mechanical compression which effectively descales thebody to an extent which suffices for most purposes. However, thisscale-free state is only transient since the body immediately begins todevelop scale at its high temperature in the ambient oxygencontainingatmosphere.

The scale-formation is, of course, detrimental because it roughens theinterior of the workpiece and otherwise may negatively affect themetallurgical process. In addition and, possibly of greatersignificance, is the fact that the scale may obscure or hide any surfaceflaws on the workpiece, thereby precluding scale is also abrasive andhard so that handling of scalecovered workpieces damages rolling-millequipment and may cause breakage until descaling is complete or someother means is taken to prevent the scale from coming into contact withsensitive equipment.

OBJECTS OF THE INVENTION It is the principal object of the presentinvention to provide an improved method of treating rolling-millworkpieces to avoid the aforementioned disadvantages and simultaneouslyextend the principles originally set out in my application Ser. No.835,359.

It is a corollary object of the invention to provide an apparatus forcarrying out the method of the instant invention and for efficientlytreating hot-rolled metal bodies.

Still another object of the invention is to provide an improved methodof and apparatus for treating a hotformed metal body to eliminate theadverse effects of scale formation both upon the body and upon theplant.

A more specific object is to provide a method and apparatus for thepurposes described which can overcome the above-mentioned difficultiesparticular to hot-rolled metal bodies.

It is another object of this invention to provide an improved method ofdetecting surface flaws in hotformed bodies and especially hot-rolledsteel workpieces.

Still a further object of the invention is to provide a method ofprotecting hot-rolled steel bodies against scaling and, therefore,improve markedly the quality of hot-rolled steel obtainable from arolling mill.

SUMMARY OF THE INVENTION These objects and others which will becomeapparent hereinafter, are attained in accordance with the invention, ina method of treating a hotrolled metal body, preferably a hot-rolledsteel workpiece, as it emerges in a hot, solid and relatively scale-freecondition from a hot-forming stage, specifically a rolling mill. Asdescribed in the aforementioned application, the invention involvespassing the workpiece directly into a treatment chamber, i.e. without anintervening sojuorn in an oxidizing atmosphere, a powder being appliedto the surface of the body in this chamber to produce an enamel-likefilm. The powder consists of a mixture of reducing agents (to remove anyslight scale whilch may remain by chemical action) and glass-formingsubstances which fuse at the temperature of the body.

According to the invention, this powder is blown against the exterior ofthe body as it passes through the chamber, whereupon the powder fuses toform a liquid film around the body which is both thin and continuous; oncooling of the body, the film solidifies to form the glassy enamel-likecoating which ruptures in the region of surface flaws on the body,thereby enabling their observation. The enamel-like coating also sealsthe surface of the body from the atmosphere to prevent any furtherscaling.

In accordance with another aspect of the invention, the method involvesthe steps of hot-forming a steel body (hot-rolling) prior to anysubstantial formation of scale thereon or after an initial descalingtreatment of the hot slightly scaled body with a reducing agent. It hasalready been noted that the enamel-like film should be thin andsubstantially translucent or transparent. When the coating hardens, itnot only seals the surface against chemical attack and further scaleformation, but itself is capable of indicating the presence of surfaceflaws as noted below. These flaws, not readily discernible otherwise,can be eliminated by conventional techniques.

The treatment chamber, according to still another aspect of theinvention, is fitted with at least one powder-spray or powder-dispensingnozzle which is connected through at least one blower to a hoppercontaining the powder described above. This noule is formed with aventuri restriction and a pipe communciating with the source ofcompressed air. This pipe, which may also be sprayed with the reducinggas or an inert gas, serves to accelerate the flow of fluids through thechamber and projects the powder out of the nozzle with greater force.

The powder can be a mixture of oxides of silicon, magnesium, aluminum,calcium and iron along with metallic sodium, boron or potassium, or acompound thereof. The use of these elemental metals has, of course, theadvantage that they act as reducing agents and will not only make acontribution to the formation of the enamel or glass layer, but will actto strip any residual scale from the surface of the workpiece. Othercompounds have been found to be satisfactory and compounds such as zincborate are most satisfactory in that the breakdown of the zin compoundyields a zinc coating on the metal body. Hence the glassformingsubstance, in this case, consists of a material adapted to leave aresidue of a permanent coating substance. The coated and cooled bodycan, of course, be stripped of its temporary glassy coating, forinstance in a straghtening station, and may subsequently be recoated forgreater permanence by spraying with paint, lacquer, synthetic-resinprotective coatings or the like. The method is quite advantageous inthat the glassy coating may be removed quite easily, i.e. more rapidlyand conveniently than the customary scale, so that the entire body canbe completely processed in one continuous operation.

Prior to the application of the enamel coating, it is possible to treatthe body with a reducing agent such as hydrogen or methane to reduce anyslight scale which may have developed thereon upon emergence of theworkpiece from the rolling mill. Of course, where reduction is carriedout by the reaction of a gas-phase reducing agent with the solid phase,there is no need to mix reducing agents, such as the powdered reducingmetals, with the glass-forming powders.

In an alternative embodiment of the invention, the' body (which can be acontinuous rod, wire, sheet or the like), is simply passed through afluidized bed of the above-described metallic and metal-oxide powders tocoat the workpiece. In any case, the coating must be thin enough torupture in the region of any flaws or to allow the flaws to be seenthrough the coating while being tough enough not to flake offimmediately as the workpiece is transported.

Thus, a key feature of the present invention is the formation of aglass-like coating upon the surface of a hot-formed body while it isstill in the heated condition and susceptible to scaling, of arelatively thin and brittle character upon cooling. This enamel layer,which may be composed of any conventional enameling composition,dispensed in the liquid or solid state and merely solidifiable on thebody and/or fusible thereon utilizing the heat of the body, surprisinglyis capable of providng a visible indication of surface flaws or defectsin the body. While the mechanism of this system is not fully understood,it would appear that the enamel film ruptures, fractures or breaks inthe presence of surface flaws, e.g. pits, striations, projections,contamination areas, cracking and similar defects, either as aconsequence of thermal stress during cooling, or mechanical movement ofvarious portions of the defect area, etc. or as a result of an inabilityto adhere effectively to these regions, thereby producing a fracturezone in the region of the defects. This marking or indication of thedefects permits the defective material to be removed or otherwisemonitored to ensure that the finished product will be free from theflaws. The enamel layer does not remain as tenaciously upon the body aswould a slag layer or other scaleforming material, but rather can bebroken away with ease as will be apparent hereinafter.

While reference has been made above to enamel and glass-like material ascapable of fulfilling the requirements of the present invention, it maybe noted that numerous enamel compositions which are capable of meltingat a wide range of temperatures including the temperatures at which themetal (steel) body emerges from the hot-forming stage, are known and maybe used in accordance with the present invention as long as the materialis applied to the body with a vicosity sufficiently low to enable theliquid material to flow into a uniform thin film coating the body. Thisfilm, which may have a thickness of the order of microns, e. g. 5-10microns, may either be applied in the form of a liquid coating or glassor may be applied in a powder stream, e.g. designed to fuse or flowalong the surface at the temperature of the body, but in either casewill be a glassy substance.

When powders are used, it is preferred to direct them against the bodyin high-velocity aerosol-type jets, the velocity being controlled bycompressed air. At sufficiently high velocity, the powder effectivelypenetrates the surface skin of the metal, which can be assumed to besubstantially free from scale, and is rapidly heated by its intimatecontact with the metal to the melting point of the powder andthereabove, whereby the molten droplets coalesce to form a liquid glassfilm upon the surface of the metal. Preferably, this treatment iscarried out in a tubular chamber enclosing the body and open at its endsto permit the body to pass through the chamber. The latter is providedwith a configuration geometrically similar to that of the body, i.e. achamber of circular cross-section when the body is a continuously castrod of circular cross-section.

As noted earlier, reducing agents may be used in the treatment processto eliminate any scale which may have been formed on the body prior tocoating with the enamel layer. Thus, the body may be treated with areducing gas or reducing agents, e. g. sodium, potassium and likereducing metals may be employed, the reduction products and the oxidizedmetal compounds resulting from the reducing reaction being incorporatedin the subsequently formed enamel coating. In this respect, it may beobserved that scale layers, when composed of the slag former present inmetallurgical systems are of a glassy nature and are soluble in theglass-like enamel coating applied in accordance with the presentinvention. I

In applying the powders, preferably in the form of a powder mixture ofthe character set forth above, it has been found to be advantageous toprovide a tandem array of blowers, the first receiving a mixture ofpowders and air and constituting a loosening means whereby the powdersare dispersed in the air stream to form a flowable mixture. In thesecond or downstream blower, the powder/air mixture is accelerated to ahigh velocity and propelled to a discharge nozzle trained upon thehot-metal surface. Between this nozzle and the second blower, there maybe provided a venturi injector to which compressed air is fed toregulate the velocity of the powder/air mixture directed against theworkpiece surface.

I have now found, further, that the workpiece can be prepared to acceptthe enamel coating to a greater extent and can be treated effectivelywithout interruption of the coating process and the rolling operation ifa jet of high-pressure water is directed (trained) upon the hotworkpiece surface, preferably perpendicularly thereto. Surprisingly, thehigh-pressure jet, which is ap plied immediately upstream of thetreatment chamber and the application of the enameling coating, operatesto remove all traces of scale and prepares, in ways which are notcompletely clear, the surface of the workpiece to better receive theenameling layer without substantial cooling of the workpiece and withoutscale formation resulting from the fact that water is considered anoxidizing agent in some metallurgical processes.

The water-jet system may be used in place of the mechanical or chemicaldescaling mentioned earlier or in addition thereto. More specifically,it has been found that all traces of scale, oxide coatings and the likeare removed, surface defects or flaws rendered more visible and theenamel coating applied with greater uniformity when the workpiece issubjected to a highpressure water jet from one or more nozzles trainedperpendicularly to the workpiece surface, the water having a pressure atthe nozzle of at least 100 atmospheres gauge and preferably of about 200atmospheres gauge. The volume rate of flow of the water is in excess of200 liter/minute, preferably about 400 liter/minute, and the nozzleaperture is circular with a diameter of 0.2 to 3mm, preferably about2.1mm. Another critical parameter is the water velocity which shouldrange from to I00 m/second and preferably is about 80 meter/second atcontact with the workpiece.

The highpressure water not only mechanically breaks up and removes thescale but also appears to etch or otherwise modify the surface so thatthe effects become more visible upon application of the enamel coatingof glass-forming or slag-forming materials. The surface of theworkpiece, as it enters the treatment chamber, is totally free fromscale and is particularly receptive to the enameling film. Furthermore,no substantial cooling takes place and the powder may be applied withoutconcern that the temperature of the workpiece has been lowered. Dryingis substantially instantaneous so that, in this respect, the powder isap plied to a clean dry surface.

DESCRIPTION or THE DRAWING The above and other objects, features and advantages will become more apparent from the following description,reference being made to the accompanying drawing, in which:

FIG. 1 is a perspective view of a rolling mill embodying the principlesof the present invention;

FIG. 2 is a vertical section through the rolling mill of FIG. 1;

FIG. 3 is a perspective view of a continuous casting installationembodying the principles of the present invention;

FIG. 4 is a vertical section through the installation of FIG. 3;

FIG. 5 is a vertical section through another apparatus embodying thepresent invention; and

FIGS. 6 and 7 are sections through a metal body at two separate stagesin its treatment according to the present invention.

SPECIFIC DESCRIPTION As seen in FIGS. 1 and 2, a slab or sheet 3 isformed between two rolls 23 and 23 of a last rolling stage 1 of arolling train. This slab 3 emerges from between these rolls 23 and 23 ina very hot, solid and relatively scalefree condition. It is transportedby driven rollers 2 through a chamber 22 where it is subjected to thereduction effects of a reducing gas, e.g. hydrogen, carbon monoxide,methane or mixtures thereof, to remove any slight scale formations whichmight have come into being thereon.

The slab 3, still hot, then enters an inlet 4a of a treatment chamber 4.As it passes through this chamber 4 it is contacted on all its surfacesby a powder P which is a mixture of reducing-metal and metal oxideparticles that fuse on the surface and form a very hard, inflexibleenamel-like glassy coating (see FIG. 6). In addition, other substances(e.g. zinc) needed for the later surface treatment of this body can beadded to this powder in the form of the material itself or as anothersubstance (e.g. zinc borate) which produces the desired coating oralloying substance in situ.

These powders are held in a hopper 11 which is closed by a vane 17 andconnected through a conduit 12 to a funnel 26 feeding a first aeratingblower 9 in turn connected to a second driving blower 10. A conduit 13leading from this second blower 10 is formed with a venturi restriction15 just downstream from the end of a narrow pipe 14 into whichcompressed air is fed. Thence the aerosol particles P are expelledthrough a nozzle 8 onto the slab 3 all around the latter. Thus anothernozzle 18 can be connected also to the blower 10 and the pipe 14 tocause the fine particles P to impinge against the underside of the slab3 in a region 27 free of rollers 2.

After the treated slab 3 exits through an outlet opening 4b of thechamber 4, it passes along a region 5 where it cools and the coatingbecomes very hard. FIG. 6 shows the slab 3 with its hardened coatings C.

Further downstream the slab 3 passes through a straightening station 6where three rollers 28a c bend the slab 3 sufficiently to breakfree thecoating C. Due to the regularity of this coating, it is easily removed.This coating is, however, very tight on the metal body 3, since thesurface of the body 3 reacts to some extent with the wetted powders P toform the coating C.

Finally, a coating station 7 having a plurality of nozzles 7a sprays amore flexible protective coating M, e.g. paint, upon the surface of thecooled slab 3. This coating M is shown in FIG. 7.

Because of the thinness of the coating C, any flaws (as shown at F inFIG. 6) will not be covered; the coating C will break in the region ofthese flaws F. Thus, an observer in the region can clearly see them andmark the slab 3 in this region so that it can be later discarded.

In this manner, the flaws are easily recognized and there is no need todo a major descaling of the sheet 3, since any slight scale is removedby the gas in the chamber 22 or the oxides sprayed against it in thechamber 4 and, for the rest of the cooling time, the sheet 3 is coveredby a very tight, nearly continuous glassy coating. Removal of thiscoating C has been found to be very simple once the sheet 3 is fullycooled and further scaling is no longer a problem.

FIGS. 3 and 4 show an alternative embodiment of the present invention.Here, molten metal is poured from-a ladle 16 into a funnel 1a on top ofa mole l of a continuous casting installation. A metal body in the formof a thick wire 3' leaves the base of this mold 1' and is bent through90 by driven rollers 3 of hyperbolic profile and through a chamber 22'similar to the chamber 22 of FIGS. 1 and 2.

A box-like chamber 4 is equipped with two nozzles 18' and 8' connectedthrough a powder-aerating apparatus to a hopper 11 just as in FIGS. 1and 2, common reference numerals referring to common structure. Thiswire 3 is here coated according to the method and principles set out inconnection with FIGS. 1 and 2.

As the wire 3' leaves the box 4', it is gripped between rollers 5'.

In this embodiment, the enamel coating is not stripped off the wire 3,since it serves to protect the wire during shipping and handling, andrelatively easily removed when desired. This coating also serves toaccentuate and make more visible flaws in the surface of the wire 3, asdescribed above for FIGS. 1 and 2.

FIG. 5 shows a further embodiment of the invention having in this case atreatment chamber 4". Here a fluidized bed is formed around a continuousworkpiece 3" riding on rollers 2". The workpiece 3" is very hot andrelatively scale free. A blower 21 having an output connected through aconduit b to the bottom of the chamber 4" and an input conduit 20aconnected to the top of this chamber 4" serves to fluidize minutemetallic and metallic oxide particles P fed into the chamber through aconduit 11a" connected to a hopper 11" above the chamber 4". In this waythe workpiece 3" is very efficiently covered by the powder, so that itcan move at high speeds through the chamber 4" and still be adequatelycoated.

SPECIFIC EXAMPLES EXAMPLE A A steel body is hot rolled and enters thetreatment chamber at a temperature (900C) somewhat below its meltingpoint. It is sprayed with a mixture of powders such that substantiallyequal parts of Si0 A1 0 Fe,0 Ca0, Mg0, Na, K and B mixed with andcarried by air strike the surface of the body. This mixture reduces theslight scale on the surface and fuses to form a glassy enamel-likecoating, over the entire exterior of the body, the fused mixture flowingto cover all of its surface to a thickness on the order of say 5 to 10microns. As the body cools, the enamel coating breaks in the region ofany flaws on it.

EXAMPLE B A steel wire is formed by hot rolling and enters a treatmentchamber as above. In this chamber it is sprayed with a powder whoseprime consitutuent is zinc borate. The hard, enamel-like coating isformed as above. Then the body is fed through a straightening apparatuswhich removes the coating and is found to be zinc coated in the sensethat zinc is alloyed with surface zones of the metal body.

In all of the aforedescribed embodiments, a water jet arrangement isprovided as described earlier. In FIGS. 1 and 2, for example, there isshown a high-pressure water-spray arrangement 30 having nozzles 31trained at the workpiece immediately upstream of the treatment chamber4. Each of the nozzles has a discharge aperture with a diameter of 0.2to 3mm as stated above and the apparatus 30 may be provided on oppositesides of the sheet-like workpiece 3 and may each include a high-pressurepump designed to project the water from the nozzles with a pressureupwards of 100 atmospheres gauge and a velocity of 15 to 100 m/sec. Acollecting rough 32 may beprovided below the workpiece to collect thewater dispensed by the nozzles and return it to the pumps of thehigh-pressured jet installation 13.

EXAMPLE C The steel body of Example A, after hot-rolling at atemperature below its melting point and above about 700C is treated withhigh-pressure water sprayed from nozzles uniformly spaced along theupper and lower surfaces of the flat body which has the configurationillustrated in FIGS. 1 and 2. Each of the nozzles has a circularaperture of a diameter of 2.1mm and is spaced between I and 5 cm fromthe workpiece. The water is ejected from the nozzles at a pressure of200 atmospheres gauge and a velocity of m/second. The steel body thenpasses into the treatment chamber in which it is sprayed with a mixtureof powders consisting of substantially equal parts by weight of silicondioxide, alumina, calcium oxide, magnesium oxide and boric anhydride toform a glassy film which, when cooled, has a thickness of 8 microns. Itis found that this film reveals surface flaws as described with respectto Example A without any obscuration resulting from residual scale. Someobscruation of the surface flaws is present when the reducing metals ofExample A are omitted from the powder mixture. With respect to FIGS.3-5, which deal with the formation of a continuously extruded rod orwire, a spray arrangement 30 with nozzles 31 is provided here as wellimmediately ahead of the treatment chamber.

EXAMPLE D The method of Example B is followed except that water isdirected at the wire from two nozzles disposed at diametrically oppositesides of the wire and having apertures of 2.1mm. The nozzles are spacedat about 3cm from the wire and direct the respective water jets againstthe workpiece with a velocity of 80 m/sec. at a pressure of 200atmospheres gauge. Again, an improvement in surface-flaw detection isobtained over the system of Example B.

The improvement described and illustrated is believed to admit of manymodifications within the ability of persons skilled in the art, all suchmodifications being considered within the spirit and scope of theinvention except as limited by the appended claims.

I claim:

1. A method of accenting and removing surface flaws in a hot metal bodyemerging from a forming stage comprising the steps of:

progressively advancing said body along a trans-port path;

applying a glassy coating to the surface of said body by continouslydepositing on said surface of said body as it emerges from said formingstage a glassforming substance fusible at least at the tempera ture ofsaid body to form an enamel-like coating thereon;

directing at least one high-pressure jet of water against said bodyimmediately prior to the application of said glass-forming substancethereto to erode residual scale and prepare the surface of said body forreceiving said enamel-like coating; selectively cracking said coating inthe region of surface flaws of said body by cooling said coating on saidbody; and removing said flaws from said body directed against said bodyat a volume rate of flow in excess of 200 I/minute.

5. The method defined in claim 4 wherein said jet is projected againstsaid body at a volume rate of flow of about 400 I/min.

6. The method defined in claim 4 wherein the jet has a diameter between0.2 and 3mm.

7. The method defined in claim 6 wherein said jet is directed at saidbody from a nozzle having an aperture diameter of about 2. 1mm.

8. The method defined in claim 6 wherein said jet is directed at saidbody with a velocity of 15 to 100 m/sec.

9. The method defined in claim 8 wherein said jet is directed againstsaid body with a velocity of about m/sec.

I 10. The method defined in claim 1 wherein said coating is formed inpart by interaction between said substance and said body.

11. The method defined in claim 1 wherein said substance consists atleast partially of a material desirable for later surface treatment ofsaid body.

1. A method of accenting and removing surface flaws in a hot metal bodyemerging from a forming stage comprising the steps of: progressivelyadvancing said body along a trans-port path; applying a glassy coatingto the surface of said body by continously depositing on said surface ofsaid body as it emerges from said forming stage a glass-formingsubstance fusible at least at the temperature of said body to form anenamel-like coating thereon; directing at least one high-pressure jet ofwater against said body immediately prior to the application of saidglass-forming substance thereto to erode residual scale and prepare thesurface of said body for receiving said enamel-like coating; selectivelycracking said coating in the region of surface flaws of said body bycooling said coating on said body; and removing said flaws from saidbody at re-gions thereon visually accented by cracks in said coating. 2.The method defined in cliam 1 wherein said jet has a pressure in excessof 100 atmospheres gauge.
 3. The method defined in claim 2 where saidpressure is about 200 atmospheres gauge.
 4. The method defined in claim2 wherein said jet is directed against said body at a volume rate offlow in excess of 200 l/minute.
 5. The method defined in claim 4 whereinsaid jet is projected against said body at a volume rate of flow ofabout 400 l/min.
 6. The method defined in claim 4 wherein the jet has adiameter between 0.2 and 3mm.
 7. The method defined in claim 6 whereinsaid jet is directed at said body from a nozzle having an aperturediameter of about 2.1mm.
 8. The method defined in claim 6 wherein saidjet is directed at said body with a velocity of 15 to 100 m/sec.
 9. Themethod defined in claim 8 wherein said jet is directed against said bodywith a velocity of about 80 m/sec.
 10. The method defined in claim 1wherein said coating is formed in part by interaction between saidsubstance and said body.
 11. The method defined in claim 1 wherein saidsubstance consists at least partially of a material desirable for latersurface treatment of said body.